Enzymatic addition of methyl groups (CH3-) to the DNA molecule is referred to as DNA methylation. In mammals the methyl groups are almost exclusively added to cytosines at CpG dinucleotides (Figure 1). Specific genomic regions have a high density of CpG dinucleotides and are defined as CpG islands (CGI). When CGIs are located in promoter regions DNA methylation typically acts to repress gene transcription (Figure 2).
Over 60% of human protein coding genes contains a CGI in the promoter and can potentially be regulated by DNA methylation. DNA methylation regulates processes, such as imprinting, X-chromosome inactivation, transcriptional repression of transposable elements and aging.